The incoming webhook URL as given by Discord. Create a webhook by following the official Discord documentation. You can use the same on all your netdata servers (or you can have multiple if you like - your decision).

One or more Discord channels to post the messages to.

Set them in /etc/netdata/health_alarm_notify.conf (to edit it on your system run /etc/netdata/edit-config health_alarm_notify.conf), like this:

###############################################################################
# sending discord notifications
# note: multiple recipients can be given like this:
# "CHANNEL1 CHANNEL2 ..."
# enable/disable sending discord notifications
SEND_DISCORD="YES"
# Create a webhook by following the official documentation -
# https://support.discordapp.com/hc/en-us/articles/228383668-Intro-to-Webhooks
DISCORD_WEBHOOK_URL="https://discordapp.com/api/webhooks/XXXXXXXXXXXXX/XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
# if a role's recipients are not configured, a notification will be send to
# this discord channel (empty = do not send a notification for unconfigured
# roles):
DEFAULT_RECIPIENT_DISCORD="alarms"

You can define multiple channels like this: alarms systems.
You can give different channels per role using these (at the same file):